1. Field of the Invention
The present invention relates to a projection device and a projection method capable of forming an image of primary color components in a time division manner by use of a white light source and a color wheel to thereby project a color image.
2. Description of the Related Art
In a projection device of a digital light processing (DLP) (registered trademark) system, each primary color component of light from a white light source is transmitted in a time division manner via a member which is referred to as a color wheel and which is provided with at least primary color filters of red (R), green (G) and blue (B), and the primary color light is successively reflected in the time division manner by an optical modulation element for displaying a monochromatic image. In consequence, an optical image is formed and projected, whereby a color image is projected and displayed.
In this type of projection device, a high-pressure mercury vapor lamp which performs arc discharge is used as the white light source, and the mercury vapor lamp is driven with an alternating current. A rotation frequency of the color wheel is set to be integer times as much as a driving frequency of the white light source.
FIG. 4 illustrates a constitution of a general color wheel 1. In the drawing, an arrow r indicates a rotating direction. Here, the wheel 1 employs a constitution of four segments in total in which a white (W) (practically transparent) segment is disposed in addition to the red (R), green (G) and blue (B) segments.
Rotation driving of the color wheel 1 is synchronized with a frame frequency of an image to be projected, and it is set to 50 (PAL)/60 (NTSC) [Hz] in many cases.
Driving (discharge) of the lamp which is the light source causes a problem such as flicker of the image, if the driving of the lamp is not synchronized with this frame frequency of the image. Therefore, the general light source lamp is always driven at 50/60 [Hz].
FIG. 5 shows a driving timing of the general color wheel in response to a driving period of the light source lamp. In this drawing, the frequency of the light source lamp is set to 60 [Hz]. On the contrary, a case where the driving frequency of the color wheel 1 is doubled to 120 [Hz] will be exemplified.
During a first half, i.e., 1/120 [sec], of one period (= 1/60 [sec]) of driving of the lamp, the light source lamp is driven with one polarity, for example, a plus (+) polarity. In synchronization with this, the segments R, G, B and W of the color wheel 1 are inserted into an optical path in this order.
Afterward, during a latter half, i.e., 1/120 [sec], the light source lamp is driven with the other polarity, for example, a minus (−) polarity. In synchronization with this, the segments R, G, B and W of the color wheel are similarly inserted into the optical path in this order.
Thus, it is appropriate to set the rotation frequency of the color wheel to be integer times as much as the driving frequency of the lamp. On the other hand, to realize a brighter projected image, a lamp having a high emission luminance is demanded.
However, the driving frequency of the lamp depends on a structure and a performance of the lamp. In a case where a structure having the high emission luminance is simply employed, there is a possibility that the lamp is required to be driven at a frequency which deviates from the synchronization frequency.
However, when the lamp is driven outside a synchronization frequency (e.g., 50/60 [Hz]), there occurs a large problem of image quality such as the flicker as described above. As a result, even if such a lamp has the high emission luminance, the lamp cannot be employed in a projector of the DLP system.
Moreover, another technology has been considered in which the driving frequency of the lamp is set to be, for example, 0.75 time as much as the rotation frequency of the color wheel. As understood from the above, the rotation frequency of the color wheel is not necessarily set to be integer times as much as the driving frequency of the lamp (e.g., Jpn. Pat. Appln. KOKAI Publication No. 2003-162001).
However, even in the technology described in the patent document, a period of each segment of the color wheel (an occupying angle of each segment on the color wheel) is influenced by the driving frequency of the lamp. Therefore, the period of each segment of the color wheel cannot arbitrarily be set.